1. Field of the Invention
This invention relates to welding wires used for automatic welding or semi-automatic welding of carbon steel or stainless steel and, more particularly, to welding wires capable of improving wire resistance weldability in manufacturing or productivity, and also improving wire feedability in welding.
2. Description of the Prior Art
In the manufacture of welding wires, generally, drawing is interrupted every lot of raw material. When the drawing is restarted, the wire of the following lot is joined to the wire of the lot which was drawn till then to omit the wire passing procedure through each drawing die of this lot. If the wire should be broken in the middle of drawing, it is necessary to quickly join the broken part to restart the drawing. Further, when a welding wire is wound onto a spool or housed in a pail pack, joining of wires of different lots is frequently performed in order to improve the yield of product. In such a joining of wires (wire welding), mechanical caulking by use of a sleeve, joining by resistance welding, and combination of the both are conventionally employed.
It is disclosed, for example, to mutually join wires by inserting the end parts of the wires to a sleeve having an inside diameter slightly larger than the outer diameter of the wire, and mechanically caulking them from above the sleeve (Japanese Utility Model Laid-Open No. 6-69411). Although the use of this method never causes a dispersion in welding strength nor requires any high skill for the joining work, the joined part can not be drawn as it is since the outer diameter of the sleeve is larger than the wire diameter.
It is also proposed to mutually connect two wires by reducing the outer diameter of the end parts of the wires to insert them to a sleeve having an outer diameter substantially equal to the outer diameter of the wires, and spot welding the sleeve to hoops in the wire end parts (Japanese Patent Application Laid-Open No. 7-116893). When the wires are mutually joined by this method, the joined part can not be a commercial product as welding wire since the chemical composition is differed between the sleeve and the wires.
Therefore, in the mutual joining of welding wires, generally, resistance welding is most frequently used because of its low equipment cost and satisfactory appearance and strength of the joined part. As the resistance welding method of welding flux-contained wires, it is disclosed to pressurize and crimp the wire end part after removing the flux involved in the wire in this part, and resistance weld the tip part formed of only the metal hoop component (Japanese Patent Publication No. 5-1118). It is also known to remove the flux involved in the wire in the wire end part, and fill Ni powder into this end part instead of the flux followed by resistance welding (Japanese Patent Application Laid-Open No. 6-262392). When the resistance welding is executed after the flux is removed as described above, a welded product of constant quality can be provided, but a problem of complication of welding work arises.
The resistance welding of wires is mostly executed by polishing the end surfaces of two wires to be joined, nipping the end surfaces of the wires by electrodes so as to be opposite to each other to contact pressurize the both, and also applying a current thereto. When the current is applied to the two wires, the contact pressurized wire end surfaces and the wires around them are heated and molten by the Joule heat by the current-carrying. When the current is I, and the electric resistance of the part to which the current is carried is R, the Joule heat W is calculated by the following mathematical expression 1. EQU W=I.sup.2 .times.R
The current I is generally alternating current, and the Joule heat W corresponds to the value obtained by multiplying the square of the ac I by the resistance R of a weld. When the wire end surfaces are arranged so as to be mutually opposed with pressure, and the both are joined together by resistance welding, the resistance R is the highest in the contact of the wire end surfaces. Thus, the heating is started first in the wire end surfaces.
The contact of the wire end surfaces is instantaneously molten because its heat capacity is extremely small to the calorific value, and the two wires are pressurized, whereby the molten metal is carried and discharged to the circumferential parts of the wires. The discharged initial molten part is generally cut and removed by a welding worker. The wire joining by resistance welding is widely used as general joining technique since the wires can be easily joined together in an extremely short time at low cost.
However, this joining largely depends on the skill of the worker, and has a problem in that a weld of high quality can not be constantly formed. For example, selection of cut form of wire end surface, pressurizing force, welding time or the like depends on the experience of the worker. In the joining of welding wires drawn to a product diameter by resistance welding, particularly, the wettability of the weld metal not only on the weld surface but also near the weld part with the wire surface has a significant influence on the product performance as welding wire. When the wettability between the weld metal and wire surface is poor, it becomes necessary to sufficiently file the surface, resulting in a remarkable reduction in efficiency of the wire welding.
This filing often scratches a sound wire surface which requires no filing, and such a damaged wire surface makes the wire feedability or electric conductivity unstable in the welding by use of this wire. Further, the poor wettability of the weld metal near the weld part causes the breaking of the wire, a feeding failure, and an unstable arc in the actual welding by use of this wire. According to the recent improvement in quality of welding wires, the welding uniformity of the weld part which was not a problem in the past is also required to be improved.
When wires are mutually welded only for joining purpose, the current I is properly selected for resistance welding according to the kind of wire such that whether the wire is for soft steel or for stainless steel, or whether the cored flux is present or not since the resistance R of the wire is changed depending on the chemical composition, dislocation density and flux ratio of the wire, whereby the wires can be easily joined. However, when the performance as welding wire is also taken into consideration, with respect to the part joined by resistance welding, the control of only the resistance welding current is not sufficient, and an improvement in wettability between weld metal and wire surface is an essential condition in order to provide an excellent performance as wire.
Further, characteristics desired as welding wire include satisfactory wire feedability. A number of attempts were made to improve the wire feedability in the past. The improvement in wire feedability by adhering sodium salt or potassium salt of a higher fatty acid such as stearic acid, oleic acid, linoleic acid, linolenic acid or the like to the wire surface and applying a lubricating oil thereon after finish drawing (Japanese Patent Application Laid-Opened No. 1-166898) and the improvement in wire feedability by retainably adhering an oily lubricant containing sodium carboxylate or potassium carboxylate onto the wire surface (Japanese Patent Application Laid-Open No. 2-284792) are known. In these method, proper quantities of alkali salt of higher fatty acid and lubricating oil are adhered to the wire surface, whereby the wire feedability is improved. Since the metal salt and lubricating oil are likely to be peeled from the wire surface and accumulated within a spring liner when the welding work is executed over a long time, the oil is deposited in the spring liner, and the wire feedability is consequently lowered. Such a peeling is resulted from the weak bonding strength of the wire surface with the higher fatty acid salt and the lubricating oil.